Reading your comment I remembered a story I heard about Jimmy Reed. Who knows if it's true, but the story was that he wasn't sharp enough to remember the words to his own songs, and his wife had to stand behind him on stage and whisper to him to remind him. Hell of a blues player though
McCartney says in an interview about the early days of the Beatles that they used to compose and rehearse new songs in hotel rooms with no recording device or paper, and the goal was to be able to play the new songs the day after.
He says (quoting from memory): "if we couldn't remember our own songs how could we expect audiences to remember them?"
If a song is hard to remember it may have a hard time becoming a hit.
He could also be someone like me where it is just very difficult to separate the voice and words of a singer from the instruments. If I listen to a song and do not actively try to decipher what the singer is saying it becomes just another instrument.
He says (quoting from memory): "if we couldn't remember our own songs how could we expect audiences to remember them?"
If a song is hard to remember it may have a hard time becoming a hit.